1. Field of the Invention
The present invention relates to press-formed containers having excellent press-formability, sterilizing property by retorting, adaptability of being heated by direct flame, as well as excellent workability.
2. Description of the Prior Art
A container containing food that can be heated by direct flame prior to eating can be represented by, for example, the one made of an aluminum foil that is press-formed into a desired shape and is fitted with a closure. This container, however, has a low rigidity, easily deforms, and permits the content to be scorched and stuck to the container.
The containers made of such a base material as a metal foil or the like are usually provided with a protection coating composed of a variety of thermosetting resin paints and thermoplastic resin paints in order to impart surface protection and functions. Among such paints, the thermoplastic paints have an advantage that no baking is required for curing the resin but have a defect in that they have poor heat resistance when they are formed into a coating. The thermosetting paints, on the other hand, have excellent heat resistance compared to the thermoplastic paints but are still defective in regard to heat resistance, resistance against extraction, or resistance against scorching.
As paints for cans that exhibit excellent resistance against hot water and corrosion, there have been known a variety of epoxy-phenol paints, epoxy-amino paints, and epoxy-acrylic paints that have excellent adhesiveness to the base materials such as of metals, as well as resistance to the heat and hot water capable of withstanding the sterilization by retorting. When the container containing content is put into an oven toaster, a microwave oven or directly on the flame to cook it by heating, however, the coated film is softened, the components of the coated resin elute out into the content and are often scorched. One of the reasons is that though the epoxy resin component exhibits excellent adhesiveness to the base material as well as excellent reactivity to other resin components, it has a tendency to be softened even at a relatively low temperature, and that the cured film still contains uncured components of relatively low molecular weights.
It has been proposed to use polyimide-type paints such as a polyimide, a polyimideimidazopyrolone, and the like as paints for being applied to the inner surfaces of the containers that will be heated for cooking. However, although these paints exhibit excellent heat resistance, they are considerably expensive and require special solvents. Therefore, these paints are not utilizable for the inexpensive containers that are to be heated for cooking.
Moreover, there have been known fluorine-type resin paints such as a polytetrafluoroethylene and the like as a coating agent to be applied on the inner surfaces of such containers for cooking by heating as a flying pan and the like. However, though the fluorine-type resins exhibit excellent heat resistance and resistance against scorching, they still are expensive and adhere poorly to the material base unless a special primer coating is provided. Therefore, the fluorine-type resins are not, either, utilizable for the one-way and inexpensive containers that are to be heated for cooking.
In general, even the thermosetting paints tend to be softened to some extent at high temperatures, permitting the resin components to migrate (extracted) into the content due to the action of hot water. And by cooking, paints are also scorched due to the mutual action relative to the content that is dehydrated.
It may appear that the above problems can be solved if the curing degree of the resin components is enhanced, i.e., if the degree of crosslinking is enhanced. However, when the resin components are crosslinked to a high degree, the coated film becomes very brittle and cannot be applied to the container in the form of a coating. Or, the coated film is peeled off or is removed upon the impact when, for example, the container falls or comes into collision. Moreover, adhesiveness to the base material and corrosion resistance decrease, too. Furthermore, for such applications as protecting the one-way, disposable and inexpensible containers that will be heated for cooking, the painting resin must be a cheaply available one and that can be easily applied.
Moreover, when the container is filled with a content, sealed, and is subjected to the sterilization by retorting, an interface peeling develops between the inner coating and the metal material, or so-called under-film corrosion develops under the inner coating, permitting iron to be eluted out or resulting in the leakage due to pitting no matter how the inner coating exhibits excellent properties to some extent for the content that is heat-cooked. Thus, there has not yet been provided an inner coating that is satisfactory with regard to corrosion resistance, content preservability and flavor retentivity.